It is a generally practiced method in pharmaceutical preparation of perform film coating on various kinds of solid medicament forms such as tablets, granules, beads, capsules and the like by use of a water-soluble cellulose ether with an object of protecting the effective ingredients contained therein from atmospheric influences or increasing beautifulness in the appearance. Although certain cellulose ethers are soluble in organic solvents and a solution thereof in an organic solvent can be used as the coating solution on solid medicament forms, the use of organic solvents is disadvantageous and has problemsn in the pollution of the working environments and toxicity due to the residual amount of the solvent in the coating layer if not to mention the increase in the costs due to the expensiveness of organic solvents. In this regard, therefore, it is a trend in recent years that the process of film coating of solid medicament forms with a cellulose ether is prformed by use of an aqueous solution thereof which is directly sprayed on to the solid medicament forms.
In the following, a more detailed description is given of the film-coating of solid medicament forms by using an aqueous solution of a cellulose ether for the respective types of the medicament forms. When a sufficiently high masking effect is desired of tablets, in the first place, it was a traditional method to provide the tablets with sugar coating. In view of the low productivity of the complicated process of sugar coating, it is a trend in resent years that sugar coating is increasinly replaced with film coating of a water-soluble cellulose ether. Film-coated tablets with a water-soluble cellulose ether, however, have problems in respect of the insufficient masking effect and sliminess of the coating in the oral cavity of the person administrated with the tablets.
Solid medicament forms in the form of granules are conventionally provided with a coating of a water-soluble cellulose ether or an enterosuluble coating material. Granules coated with a water-soluble cellulose ether are usually not imparted with a full masking effect so that the person administered with the coated granules may sometimes feel bitterness in addition to stickiness in the oral cavity. In the enteric-coated granules, increase in the coating amount with an object to enhance the masking effect causes a problem in the decreased rate of releasing of the effective ingredients in the intestines.
Apart from the above, on the other hand, comsumption of solid medicament forms in the form of beads is increasing in recent years for the reasons in the medicament preparation or upgrading of medicament forms such as amplification of product assortment with the same effective ingredient. Accordingly, solid medicament forms conventionally prepared in the form of granules are sometimes converted into the form of beads or products of beads are added to the product assortment of granules. Conversion of granules into fine beads is naturally accompained by a great increase in the surface area of the solid medicament form so that solutions must be sought for novel problems or for the problems already solved for granules by undertaking a coating method with a water-soluble cellulose ether or an enterosoluble coating material in connection with the process of medicament preparation such as the great increase in the coating amount or the time taken for coating in order to obtain a masking effect of the same degree as in granules and tablets. In particular, a problem in the coating procedure of fine beads by use of a conventional aqueous solution or organic solution is the difficulty in preventing agglomeration of particles in the course of the coating works.
Further, solid medicament forms are sometimes provided with wax coating. For examples, tablets are wax-coated without using any organic solvent or solution by adding a molten wax into the coating pan or by adding a fine powder of wax into the coating pan followed by heating. The amount of wax coating is, however, usually low and rarely exceeds 0.5% by weight on tablets because the purpose of wax coating is for glazing so that no masking effect can naturally be obtained by the wax coating. The amount of wax coating on granules and beads also does not exceed 5% by weight without exception and the method of wax coating thereon is limited to an ordinary method using a solvent.
As a method for film coating with a water-soluble cellulose ether, Japanese Patent Kokai 62-91272 teaches a method based on a principle that the viscosity of an aqueous solution of a hydroxypropyl cellulose is decreased by heating the aqueous solution. This method, however, is not applicable when the aqueous solution of the cellulose ether has a high viscosity as is the case when the aqueous solution contains the cellulose ether in a high concentration or the cellulose ether has a high degree of polymerization.